This invention is directed to a system and method employing two, sequentially applied materials to facilitate the release of a molded article from the mold surface, and to a system and method which is particularly advantageous for providing a water-soluble releasing interface having a permanence permitting the manufacture of a plurality of molded articles by reaction injection molding, most particularly with reinforced and unreinforced urethane materials, between applications of the release system.
In the injection-molding of plastic parts, it is generally advisable to provide a release agent on the surface of the mold to facilitate the release and removal of the molded part from the mold. It is desirable in any injection-molding operation that a multiplicity of mold cycles is obtained between applications of the release agent. In the past, a number of release agents have been available to permit such operation; for example, such release agents include those sold by FreKote, Inc., under its trademarks.
Investigation of reaction-injection molding, now referred to as RIM, was undertaken by both chemical and equipment suppliers in the middle 1960's. The reaction-injection molding process involves the injection molding of parts from highly reactive liquid components of a chemical system. Reaction-injection molding was of particular interest in the automotive industry which, in its efforts to comply with the United States government regulations on achieving higher mileages for its automobiles and on collision standards for its bumper systems, faced the problem of reducing body weight and producing resilient front-end parts associated with its bumper systems. As a result of this developing interest in the automotive industry, many chemical companies became heavily involved in formulating systems using urethane intermediates for molding large elastomeric automotive parts. In the early 1970's, suitable machinery for reaction-rejection molding became available; and small-scale production was started.
By the model year 1975, several automobile manufacturers were manufacturing reaction-injection molded urethane parts; and an increasing demand has developed in succeeding years for the use of RIM elastomers in producing not only the resilient parts associated with automotive bumper systems, but spoilers, air dams, wheel flares, and other body parts as well. More recently, urethane elastomers have been provided with reinforcement, such as glass fibers to extend the use of the RIM process for molding exterior body parts, such as fenders and door panels. The reinforcement of the RIM elastomers has provided increased stiffness, has reduced the thermal expansion of molded parts, and has extended the weight advantages of RIM materials such as urethane to many parts of the automobile body.
While the introduction of the RIM process into commercial manufacturing operations has been pioneered by the automotive industry, the RIM process is not limited to the production of automotive parts. The continuing development and introduction of RIM molding materials extends reaction-injection molding to the production of a wide variety of non-automotive parts.
The reaction-injection molding process is experiencing growth in its application and use because of many advantages. Reaction-injection molding permits low mold pressures and low mold temperatures. The low injection molding pressures reduce the clamping force required at the mold and low molding temperatures mean significantly lower energy requirements compared with other molding processes. Reaction-injection molding requires reduced heat energy because many of the chemical reactions, for example those obtained with urethane materials, are exothermic, reducing the amount of heat that is needed to maintain proper mold temperature. Mold costs are substantially reduced with reaction-injection molding. Both thin and heavy wall sections are attainable with parts manufactured by reaction-injection molding. Furthermore, reaction-injection molding permits parts that are generally free from flow and stress lines and also free from molded-in stresses which cause warpage and stress cracking. Complex shapes and variable wall thicknesses and ribbing are readily obtainable in RIM-molded parts, and it is possible to mold very large parts in a single operation, eliminating the need for smaller parts and subassembly steps.
In spite of all its advantages, the reaction-molding process has been burdened with one continuing problem: the release of the molded part from the mold surface. The reaction-injection molding process requires the injection of highly reactive chemical intermediates directly into the mold. The mold surface and any release agent applied to the mold surface are thus exposed to the reactive chemical components of the system and to their exothermic reaction. In addition, the presence of reinforcing materials, such as glass fibers has gravely aggravated the problem by introducing their abrasive action on any release agents placed on the mold surface. The abrasive removal of release agents from the mold surface by reinforcing materials exposes the mold surface itself to the reactive components and their reaction.
This problem has inhibited the adoption of the otherwise highly attractive RIM process. It increases the time necessary to produce parts by the RIM process and thus increases the cost of the resulting parts. For example, the cycle time for the production of a single-urethane part can be on the order of 60 to 120 seconds, which is generally typical for RIM urethane-molding systems. The application of release agent to the mold surface generally takes 30 to 60 seconds of that total time. Thus, if a satisfactory release agent were avilable, the production time for RIM urethane parts could be reduced by the time required for application of the release agent. With the best release agents available prior to this invention, however, only one or two urethane parts could be produced by the RIM process before it became necessary to recoat the mold surface with a release agent. In addition, with the prior release agents, it became necessary to cease production frequently and clean the mold to remove the release agent from those areas in which it builds up.
The release agents which have been in use with reaction-injection molding have been generally waxes or soaps applied in one coating. The soaps have generally provided only one release and have generally required that the mold be coated after each cycle; i.e., after the production of each molded part. Since the soaps are not uniformly removed from the mold with each molded part, an excessive build-up of soap develops on differing areas of the mold; and it becomes necessary, from time to time, that the mold be removed from production and cleaned. The time-consuming and wasteful applications of soaps with each cycle of the mold and the removal of the mold from production for cleaning interrupt production of RIM molded parts and substantially increase their costs.
Waxes have been used in an effort to obtain more than one cycle between applications of the release agent. While waxes have generally provided more than one cycle of the mold, and thus more than one part between applications of the release agent, the parts Cobtained from wax-coated molds have required solvent washing to remove the wax from the surface.
In the automotive industry, particularly, RIM-molded parts are generally part of the body shell and must be painted. Painting, and the resulting finish, of an automotive body must be of the highest possible quality since the cosmetic appearance of the automobile body is believed to be critical to the sale of automobiles. The presence of any wax on a molded part prevents a satisfactory finish. Solvent-washing each part is costly, time-consuming, and imposes a potentially hazardous step in the manufacture of such parts.
Thus, manufacturers seeking to introduce the reaction-injection molding process into their manufacturing operations in an effort to achieve the advantages accompanying the use of such a process have been retarded prior to this invention by the problem of part removal from the mold surface and a lack of any satisfactory release agent for the RIM process. Only soaps, with the attendant cost associated with their repeated application to the mold surface and the cleaning associated with their build-up in the mold, and waxes, with the requirement for solvent cleaning of the resulting molding parts, have been available to the manufacturer prior to this invention.